When a creator (usually writers, it seems) bans or restricts their fans from writing fanfic, drawing fanart, or generally using the author's work "creatively". You can buy the books, buy The Merch and read "officially sanctioned" material, but the hounds of hell will be unleashed should you write their characters into your story. A milder form exists in the form of restriction; the creator might ban Slash Fic or PWP material, for example. Other creators are more picky, setting "rules" to which all fic/fanart creators have to adhere to. How they keep tabs on such things is a mystery.